Automatic milking systems have over the years revolutionized dairy farm operations which used to rely on hand milking to collect milk from cows. These automatic systems generally have a vacuum pressure line which extends from a teat-cup cluster fastened to a cow's udder to a milk collection line which transports the warm milk from the cow to a receiver. The receiver is generally cylindrical or cup-like in shape and separates the milk out by gravity from the vacuum to create a liquid seal at its bottom. A liquid pump conveys the warm milk into a milk cooling and storage tank where it is then cooled and stored until picked up.
To guarantee the continued quality and cleanliness of the milk, local Health Department standards almost always require some type of thorough cleaning and inspection of the system on a daily basis to remove any impurities from the system and prevent any buildup which might contaminate the new milk. This requirement is usually fulfilled by flushing the individual vacuum lines, milk collection line, receiver, and other surfaces which handle the milk with wash water maintained at or above a minimum temperature. This flushing is typically performed by circulating wash water through the system much in the same manner that milk is collected. An auxiliary wash pipeline parallels the milk collection line and extends from a wash basin filled with wash water to the end of the collection line. The milk line from the milk pump is removed from the cooling and storage tank and placed in the wash basin, to complete the wash water's return path. The system is then operated by turning on the vacuum pump and milk pump to circulate wash water through the milk collection line, the receiver and the milk pump.
Because the milk collection line is typically a long pipe extending through an unheated barn, it acts much as a radiator, making it difficult to maintain the wash water at its prescribed temperature. To ensure compliance with the minimum temperature standard and ensure adequate cleaning, it is oftentimes necessary to continuously heat the water in the wash basin with an auxiliary heater, when the recirculating flushing technique is used. This auxiliary heater is used only during the cleaning cycle, and of course, costs some money to buy and requires some additional energy to run.
Another important feature which has been developed as part of the automatic milking systems is the water chilled pre-cooler, which pre-cools the milk after it is pumped and before it enters the tank. These pre-coolers reduce the temperature of the milk from the 90.degree. to 100.degree. F. (32.degree. to 38.degree. C.) at which it is taken from the cow to some lower temperature after it passes through the milk pump. There are several important advantages to pre-cooling the milk which make it desirable to achieve this pre-cooling as quickly as possible after taking it from the cow. For example, any churning or agitation of warm milk tends to promote hydrolytic rancidity, which in turn decreases the expected shelf life, and adversely affects the taste. This is particularly true when the milk is in the critical temperature range which is close to the body temperature of the cow. Unfortunately, most pre-cooling systems presently available are designed to operate under positive pressure conditions such as exist downstream from the milk pump as opposed to vacuum conditions which exist upstream. Thus, the warm milk undergoes turbulence in the receiver and experiences severe agitation caused by the powerful pumping action of the milk pump while the milk is in the critical temperature range.
Another important advantage attained by pre-cooling is that the system avoids mixing 90.degree. F. (32.degree. C.) milk directly in with the 38.degree. F. (3.degree. C.) milk in the tank which could significantly increase the overall temperature of the milk contained therein. Typically, another standard adopted by most local Health Departments requires that the overall temperature of the milk contained in the tank not exceed a temperature of 50.degree. F. (10.degree. C.) or the like. As can be appreciated, milk with a temperature of 90.degree. F. (32.degree. C.) at a substantial flow rate can rapidly increase the temperature of the stored milk. To prevent an excessive temperature rise, refrigeration units on the milk cooler are continuously operated throughout the milking and for some longer period thereafter to completely refrigerate the milk and bring it back down to the 38.degree. F. (3.degree. C.) storage temperature. Generally, any cooled milk in a tank which exceeds the 50.degree. F. (10.degree. C.) temperature limit must be dumped or sold at a lower grade with a correspondingly lower price, thereby presenting a substantial risk each time warm milk is blended with the cold milk being stored in the milk cooling and storage tank. Furthermore, as automatic temperature recorders are often required on the tank, a dairy farm operator can undergo some psychological tension as he watches the temperature of the entire tank move up during each milking.
By pre-cooling the warm milk to some lower temperature prior to blending with the already cooled milk, this rapid rise in temperature is avoided, thereby lessening the risk of loss. Furthermore, it is more energy efficient to pre-cool the heated milk to a temperature approaching that of the storage temperature instead of dumping the heated milk into the tank and refrigerating what may be a large quantity of milk over a smaller and lower temperature range. With pre-cooling, one refrigeration unit in the pre-cooler can work on just the new milk at a higher, more energy efficient temperature range of approximately 90.degree. F. (32.degree. C.) down to 50.degree. F. (10.degree. C.). Without pre-cooling, several larger refrigeration units on the tank must work on the larger amount of milk contained in the tank at a lower, less energy efficient temperature range of approximately 50.degree. F. to 38.degree. F. (10.degree. C. to 3.degree. C.). Thus, pre-cooling offers the potential for making the overall system more energy efficient.
A drawback of the prior art pre-coolers is that they generally circulate cold well water or the like through a baffle arrangement in the milk line to achieve the refrigerating effect. However, these systems require a large quantity of water, typically as much as twice the amount of milk processed to achieve a significant temperature change. This can be a rather wasteful use of what may be a limited supply of well water, or an expensive use of municipal water. Of course, these systems are not even feasible in more arid areas of the country, including parts of Texas and Oklahoma. Furthermore, except in certain northern areas of the country, the temperature of available well water does not nearly approach the desired ultimate temperature of the milk which decreases the effectiveness of the system. Another type of water pre-cooler uses a separate refrigeration system to chill the water. However, all of the water chiller types of pre-coolers operate at a lower, less energy efficient temperature range as they rely on water in its liquid state as an intermediate cooling medium.
Some systems in the prior art have attempted to move the pre-cooling function upstream of the milk pump by providing a heat exchanger inside the receiver. The heat exchanger generally includes a plurality of horizontally disposed baffles which force the milk through a circuitous path to ensure sufficient contact to achieve the desired cooling effect. An example of one such system is shown in U.S. Pat. No. 3,271,968. Although these prior art systems do provide somewhat limited pre-cooling of milk upstream from most other prior art systems, their heat exchanger design creates additional problems which greatly limit their effectiveness.
As explained above, cleanability and inspectability are of primary concern in a milk handling system. However, with the baffling and generally horizontally disposed surfaces of the prior art receiver enclosed heat exchangers, substantial disassembly by hand is required to either clean or inspect, which can be time consuming and expensive. Furthermore, the baffling and horizontally disposed surfaces slow up the velocity of the milk through the system and inherently limit the throughput capacity of the entire system. This is a very expensive price to pay to achieve the somewhat limited pre-cooling of a water chilled system with a receiver enclosed heat exchanger, making these systems of questionable advantage over the downstream pre-coolers.
To solve the problems left unresolved by the prior art, and to meet a long felt need in the industry, applicants have succeeded in developing a receiver enclosed pre-cooler having a flow through design and an increased refrigeration capability to achieve a much higher flow rate, and which can be cleaned and inspected in place using the standard flushing technique of the prior art. Furthermore, applicants' heat exchanger may be used to heat the wash water as the system is flushed, thereby eliminating the need for auxiliary heaters previously required. Applicants' refrigerated receiver unit has a coiled plate type heat exchanger which is adapted for use with a direct expansion refrigeration system to utilize the inherently higher energy efficiency of such a system over the water chilled systems. The coiled heat exchanger is disposed in a substantially vertical orientation inside the receiver with virtually all of the heat exchange surface being vertical and parallel to the flow of the milk through the receiver. Thus, a significantly increased throughput is available as there are no horizontal baffles as in prior art designs to slow the velocity of the milk. Furthermore, these substantially vertical surfaces and flow through design permit the heat exchanger and receiving unit to be cleaned by the circulation of cleaning water therethrough, with additional agitation provided by an atmospheric valve which intermittently opens to agitate the water in the receiver against the vacuum created by the vacuum pump. The ability to be cleaned in place represents a significant advance over the prior art designs as it eliminates the time consuming and expensive disassembly and hand cleaning that would have been required with the prior art. An inspection port in the top of the receiver permits visual inspection of substantially all of the coiled heat exchanger as it remains in place which is another feature not present in the prior art designs. Prior art systems had to be disassembled to be inspected.
Applicants' heat exchanger offers another significant advantage over the prior art as the refrigeration system may be run in reverse, as a heat pump, or operated in a hot gas bypass mode or "defrost" mode to heat the liquid being circulated through the receiver. Thus, the wash water may be directly heated by the heat exchanger as it passes through the system during the cleaning cycle. This not only counteracts the tendency of the wash water to dissipate heat rapidly as it traverses the milk collecting line, but also serves to heat the very surface which is toughest and most important to clean. By reverse cycling the heat exchanger and receiver, the temperature of the wash water may be maintained well above the minimum standards set by the local Health Department to ensure that the system is thoroughly cleaned during washing. This eliminates the requirement of the prior art systems for a separate heater to be immersed in the wash basin or wash line to re-heat the water as it circulates through the system during the cleaning cycle. Thus, applicants' refrigerated receiver serves a dual purpose by refrigerating milk as it passes through the receiver and also heating wash water as it is circulated therethrough during the cleaning cycle.
Applicants' flow through, vertically disposed, coiled plate type heat exchanger also eliminates the horizontal surfaces and baffling of prior art refrigerated receiver designs which itself created a turbulence in the milk. In applicants' device, the milk contacts the plate surfaces and smoothly flows over them in a thin film which increases the effectiveness of the heat exchanger and which accelerates the cooling process. In the prior art horizontally disposed baffle systems, milk "puddles" before flowing from one baffle to another which creates an indirect flow of milk tending to trap contaminates and lead to blockages where bacteria can collect and grow, possibly causing spoiling and rancidity. This problem is exacerbated by the small orifices which lead from one baffle to another and which are offset to actually prevent the direct flow through of milk. With applicants' device, a large generally annular space is provided between successive turns which eliminates any tendency of the receiver to clog and instead will pass through all but the largest contaminants to a filter which may be quickly cleaned or changed.
There are many additional benefits and features of applicants' device and these are more fully explained in the drawings and description of the preferred embodiment which follows.